Production of electronic coatings by spin coating a partially fluorinated polyimide composition

ABSTRACT

In humid atmospheres (e.g., 40% relative humidity or above) solutions of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimides tend to be unstable in the sense that during spin coating operations undesirable precipitate formation occurs on the rotating surface of the wafer. The result is the formation of unacceptable coatings due to their irregularity and lack of uniformity. Described are solutions of these polyimide polymers in a solvent containing one or more liquid aromatic hydrocarbons having a boiling point of at least about 110° C. and one or more dipolar aprotic solvents having a boiling point of at least about 150° C., such that the solution (a) contains on a weight basis from about 5% to about 50% of the polyimide, and (b) does not undergo precipitate formation during spin coating in an atmosphere of at least up to about 55% relative humidity.

BACKGROUND

Polyimides derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride and 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane areuseful, inter alia, for the production of electronic coatings onsemiconductor wafers, such as polysilicon wafers. In order to applysolutions of such polyimides to the wafers spin coating procedures areused, and in these operations purity, integrity, and uniformity of theresultant polyimide coating are essential.

Unfortunately it has been discovered that in humid atmospheres (e.g.,40% relative humidity or above) solutions of such polyimides in somecommonly used solvents (e.g., N-methylpyrrolidone) tend to be unstablein the sense that during spin coating operations undesirable precipitateformation occurs on the rotating surface of the wafer. The result isthat the coating loses adhesion to the wafer and flies off of it duringspin coating.

The need thus exists for solutions of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimideswhich do not undergo undesirable precipitate formation on rotating wafersurfaces when used in spin coating operations even if conducted underconditions of 55% relative humidity. This invention is deemed to fulfillthis need in an effective and efficient manner.

The Invention

In accordance with this invention there is provided a partiallyfluorinated polyimide composition especially adapted for use in spincoating wafers of semiconductive materials. Such composition comprises asolution of (i) a 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimidepolymer having an inherent viscosity in the range of about 0.05 to about1.5 dL/g --as measured in N-methylpyrrolidone at room temperature (25°C.) at a concentration of 0.5 g/dL --in (ii) a solvent containing atleast 40% by weight of one or more liquid aromatic hydrocarbons having aboiling point of at least about 110° C. and at least 5% by weight of oneor more dipolar aprotic solvents having a boiling point of at leastabout 1/8° C., such that the solution (a) contains on a weight basisfrom about 5% to about 50% of such polyimide and (b) does not undergoprecipitate formation during spin coating in an atmosphere of up to atleast about 55% relative humidity. These solutions preferably contain ona weight basis from about 10% to about 35% of such polyimide. It is alsopreferable that the inherent viscosity of the polyimide (as measured ata concentration of 0.5 g/dL in N-methylpyrrolidone at room temperature)fall in the range of about 0.1 to about 0.9 dL/g.

Preferably, the polyimide is produced in the above aromatichydrocarbon-dipolar aprotic solvent blend (with or without additionalco-solvent) although if desired, it may be formed in a differentreaction medium, recovered therefrom, and then dissolved in the abovesolvent blend or component thereof (with or without additionalco-solvent). Alternatively, the polyimide may be formed in the dipolaraprotic solvent, and the above aromatic hydrocarbon may then be addedthereto, with one or more co-solvents introduced at any suitable stageof the overall operation. As is well known, polyimides are formed byreacting essentially equimolar proportions of a primary diamine with atetracarboxylic acid or derivative thereof, preferably the dianhydrideof the tetracarboxylic acid. Thus in the practice of this invention itis preferred to produce the solutions by reacting in an appropriatereaction solution an essentially equimolar mixture of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and a2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane e.g.2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane, a mixture of2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane and2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, or most preferably,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.

At some stage during or after the reaction, the solution should beheated to a suitably high temperature (usually in the vicinity of 140°C. or above) in order to convert any intermediate polyamic acid intopolyimide.

As noted above, the solvent used in the compositions of this inventioncontains both one or more liquid aromatic hydrocarbon having a boilingpoint of at least about 110° C. and one or more dipolar aprotic solventshaving a boiling point of at least about 150° C. Other solvents may becopresent provided they do not, in the concentrations employed,adversely affect the stability and desirable spin coatingcharacteristics of the resultant solution when used in an atmosphere ofup to 55% relative humidity. Generally speaking, other solvents whichmay be present in appropriate proportions include ethers, aromatichydrocarbons having boiling points below about 110° C., cycloaliphaticketones, and the like.

Examples of liquid aromatic hydrocarbons boiling above 110° C. include

1,2,3,4-tetramethylbenzene

1,2,3,5-tetramethylbenzene

toluene

o-xylene

m-xylene

p-xylene

1,2-diethylbenzene

1,3-diethylbenzene

1,4-diethylbenzene

3,5-diethyltoluene

n-butylbenzene

3-propyltoluene

4-propyltoluene

tetrahydronaphthalene

and the like, including mixtures of two or more such aromatic solvents.

Examples of dipolar aprotic solvents boiling above 150° C. include

N-methylpyrrolidone

N,N-dimethylformamide

N,N-dimethylacetamide

dimethylsulfoxide

and the like, including mixtures of two or more such dipolar aproticsolvents.

Examples of suitable co-solvents include

cyclohexanone

2-methylcyclohexanone

3-methylcyclohexanone

4-methylcyclohexanone

2,5-dimethylcyclohexanone

2,6-dimethylcyclohexanone

cycloheptanone

cyclooctanone

isophorone

tetrahydrofuran

diethylene glycol dimethyl ether

triethylene glycol dimethyl ether

dimethoxyethane

acetone

methylethylketone

and the like, including mixtures of two or more such solvents.

Liquid methylbenzenes having boiling points above 130° C. are the mostpreferred aromatic hydrocarbons for use in the practice of thisinvention. The most preferred dipolar aprotic solvent for such use isN-methylpyrrolidone.

Needless to say, the reactants and solvents used in forming thecompositions of this invention should have sufficiently high purities tosatisfy the requirements of electronic coatings. Thus the solids arepreferably recrystallized from highly pure solvents and the liquids arepreferably purified by use of distillation or other purificationtechniques.

In another of its forms, this invention provides a method of forming acoating upon a planar substrate such as a semiconductor wafer. In thismethod a suitable quantity of a composition of this invention is appliedto the central region of the planar surface and the substrate is rotatedat a speed sufficient through centrifugal effect to cause thecomposition to flow outwardly to the perimeter of the surface and in sodoing form a substantially uniform liquid coating thereon. Ordinarilyrotational speeds in the range of about 1,000 to about 10,000 rpm forperiods in the range of about 10 seconds to about 5 minutes are mostuseful, although departures from either or both of these ranges may beemployed where the circumstances warrant or justify such departures.Generally speaking, the higher the rotational speed, the thinner thecoating. Once the liquid coating has been formed over the planar surfacethe coated substrate is heated to an elevated temperature normally inthe range of about 80 to about 130° C. to dry the coating withoutdestroying its integrity. Thereafter the dried coating is heated to astill higher temperature (e.g., in the range of about 200° C. to about450° C.) to bake the coating. It has been found that if the dry coatedsubstrate is heated to a temperature of at least about 350° C., thesolvent resistance of the coating is improved.

A wide variety of substrates may be coated in this manner, includingmetals, ceramics, high temperature resistant polymers, intermetalliccompounds and compound semiconductors (e.g., GaAs, GaAs.sub.× P.sub.1-×, etc.), and the like. Usually, but not necessarily, the substratewill be in the form of a disc or wafer.

The practice and advantages of this invention are illustrated in thefollowing examples. Examples 1-3 typify the procedures that may be usedto produce the compositions of this invention. The manner by which thecompositions of this invention may be used in spin coating operations isillustrated in Example 4.

EXAMPLE 1

2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride/2,2bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimidewas produced as follows: 2,2-Bis(3,4-dicarboxyphenyl)hexafluoro-propanedianhydride (21.302 g) was added to a stirred solution of 24.926 g of2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 112.33 g ofN-methylpyrrolidone (NMP) heated to 80° C. Stirring was effected bymeans of a double spiral agitator, and the system was maintained under adry nitrogen atmosphere. An additional 19.44 g of NMP was used to rinsethe anhydride into the diamine solution. The reaction mixture was slowlyheated to 198.5° C. over a period of 2.25 hours with stirring, andduring this time 20.7 mL of a mixture of water and NMP distilled out ofthe reaction vessel. The reaction mixture --a viscous solution of thepolyimide in NMP --was allowed to cool to room temperature. A sample ofthe solid polyimide polymer (precipitated from solution by addition ofdeionized water and then thoroughly washed with deionized water anddried under vacuum) was found to have an inherent viscosity of 0.65 dL/gas measured in N-methylpyrrolidone at room temperature (25° C.) at apolyimide concentration of 0.5 g/dL. Solutions of this invention may beformed by diluting individual portions of the viscous reaction mixturewith appropriate quantities of suitable aromatic solvents such as1,2,3,4-tetramethylbenzene (TMB) and stirring the mixture for two hours.

EXAMPLE 2

Using the procedure described in Example 1, an NMP solution of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride/2,2bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimidewas produced employing reaction temperatures of 197-198° C. and a longerreaction period. The product solution was found to have a solids contentof 27.0% as compared to a calculated value of 26.7%. A sample of thesolid polyimide polymer, isolated as in Example 1, had an inherentviscosity of 0.72 dL/g as measured in N-methylpyrrolidone at roomtemperature (25° C.) at a polyimide concentration of 0.5 g/dL. Asolution of this invention (12% solids content) was formed by dilutingan 8.044 g portion of the viscous reaction mixture with 9.711 g of TMBand stirring the mixture thoroughly.

EXAMPLE 3

2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride/2,2bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimidewas produced as follows: 2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride (21.302 g) was added to a stirred solution of 24.926 g of2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 112.04 g ofN-methylpyrrolidone (NMP) heated to 82° C. Stirring was effected bymeans of a double spiral agitator, and the system was maintained under adry nitrogen atmosphere. An additional 19.74 g of NMP was used to rinsethe anhydride into the diamine solution. The reaction mixture was slowlyheated to 199° C. over a period of 2.5 hours with stirring, and duringthis time 8.4 mL (8.77 g) of a mixture of water and NMP distilled out ofthe reaction vessel. The reaction mixture --a viscous solution of thepolyimide in NMP --was allowed to cool to room temperature and found tohave a solids content of 26.6% as compared to a calculated value of26.3%. A 23.08 g portion of this solution was diluted with 100.02 g ofNMP and one-half of the resultant diluted solution was added slowly to500 mL of deionized water while continuously agitating the system in aWaring blender. The solid polyimide polymer which precipitated wasrecovered by filtration and slowly added to another 500 mL of deionizedwater continuously being agitated in a Waring blender. The so-washedpolymer was recovered by filtration, and this water-washing andfiltration procedure was repeated once again with another 500 mLquantity of deionized water. The solid polymer was then dried at 55° C.under reduced pressure (0.1 mm Hg). The polyimide was found to have aninherent viscosity of 0.78 dL/g as measured at room temperature (25° C.)in N-methylpyrrolidone at a polyimide concentration of 0.5 gram perdeciliter. A portion of this solid polymer was used to form a 20% solidssolution in a solvent composed of 80% TMB and 20% NMP.

EXAMPLE 4

Spin coating tests were carried out in order to determine the behaviorof different polyimide solutions under conditions of controlled relativehumidity. The procedure employed in these tests involved use in a roomof controllable humidity of a spin coater equipped with a rotatablevacuum chuck for holding the wafer in place in a horizontal position,and a fume exhaust system. Commercially available silicon wafers threeinches in diameter were used as the substrates for the coatings. Withthe wafer held in a horizontal non-rotating position, a quantity ofapproximately three grams of coating solution was applied to the centerof the wafer, and the wafer was then spun at 5000 rpm for one minute.During this time the wafer was subjected to visual observation todetermine the characteristics of the coating. Coatings which develop amilky white appearance (precipitate formation) or which spin off piecesof coating are unsatisfactory. Satisfactory coatings show neither suchdefect; rather, they remain clear, smooth and uniform in appearance.

The results of these tests are summarized in the following table whereinCH designates cyclohexanone, No PPT signifies that no precipitateformation occurred during the spin coating operation, and PPT signifiesthat precipitate formation occurred during the spin coating operation.The percentages shown for the solvent and solids (solids represents theconcentration of the polyimide in the solvent) are on a weight basis,and the inherent viscosities shown (which are a measure of the molecularweights of the polyimide polymers) were measured in N-methylpyrrolidoneat room temperature (25° C.) at a polyimide concentration of 0.5 g/dL,and are expressed in terms of deciliter per gram.

                  TABLE                                                           ______________________________________                                        Results of Spin Coating Operations                                            ______________________________________                                                                   In-   Re-                                                              So-    herent                                                                              lative                                                                              Behavior                               Run                 lids,  Vis-  Humid-                                                                              During                                 No.  Solvent        %      cosity                                                                              ity, %                                                                              Spin                                   ______________________________________                                        Comparative Compositions:                                                     1    100% NMP       19     1.00  30    No PPT                                 2    100% NMP       19     1.00  40    PPT                                    3    100% NMP       19     0.65  44-49 PPT                                    4    100% NMP       12     0.65  44-49 PPT                                    5    35% NMP-65% CH 12     0.65  44-49 PPT                                    6    60% NMP-40% CH 19     0.65  44-49 PPT                                    7    65% NMP-35% TMB                                                                              19     0.72  51    PPT                                    8    66% NMP-34% TMB                                                                              19     0.72  51    PPT                                    Compositions of this Invention:                                               9    32% NMP-68% TMB                                                                              12     0.72  51     No PPT*                               10   20% NMP-80% TMB                                                                              20     0.78  55    No PPT                                 ______________________________________                                         *At 56% relative humidity this solution did form a precipitate.          

While the compositions of this invention are well adapted for use inspin coating applications, they may be used for other purposes, such asin formation of coatings by spray coating or immersion techniques,formation of films by solvent casting procedures, formation ofcomposites by impregnation, and the like.

The foregoing disclosure has been presented for purposes of illustrationand not limitation. As can readily be appreciated by those skilled inthe art, this invention is susceptible to considerable variation in itspractice within the spirit and scope of the ensuing claims.

What is claimed is:
 1. A partially fluorinated polyimide compositionespecially adapted for use in spin coating wafers of semiconductivematerials which composition comprises a solution of (i) a2,2-bis(3,4-dicarb oxyphenyl)hexafluoropropanedianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyimidepolymer having an inherent viscosity in the range of about 0.05 to about1.5 dL/g (as measure in N-methylpyrrolidone at room temperature at aconcentration of 0.5 g/dL) in (ii) a solvent containing at least 40% byweight of one or more liquid aromatic hydrocarbons having a boilingpoint or at least about 110° C. and at least 5% by weight of one or moredipolar aprotic solvents having a boiling point of at least about 150°C., such that said solution (a) contains on a weight basis from about 5%to about 50% of such polyimide, and (b) does not undergo precipitateformation during spin coating in an atmosphere of up to at least about55% relative humidity.
 2. A composition of claim 1 wherein the2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane of said polyimide is2,2-bis[4-(4aminophenoxy)phenyl]hexafluoropropane.
 3. A composition ofclaim 2 wherein the solvent additionally contains at least oneadditional co-solvent that does not cause the solution to undergoprecipitate formation during spin coating in an atmosphere of up to atleast about 55% relative humidity.
 4. A composition of claim 2 whereinthe solvent is essentially a mixture of N-methylpyrrolidone and one ormore liquid methylbenzenes.
 5. A composition of claim 1 wherein saidsolution contains on a weight basis from about 10% to about 35% of suchpolyimide.
 6. A composition of claim 1 wherein the inherent viscosity ofthe polyimide as measured in N-methylpyrrolidone at room temperature ata concentration of 0.5 g/dL is in the range of about 0.1 to about 0.9dL/g.
 7. A composition of claim 1 wherein said solution contains on aweight basis from about 10% to about 35% of such polyimide, and whereinthe inherent viscosity of the polyimide as measured inN-methylpyrrolidone at room temperature at a concentration of 0.5 g/dLis in the range of about 0.1 to about 0.9 dL/g.
 8. A composition ofclaim 7 wherein the 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane ofsaid polyimide is 2,2-bis[4-(4aminophenoxy)phenyl]hexafluoropropane. 9.A composition of claim 8 wherein the solvent additionally contains atleast one additional co-solvent that does not cause the solution toundergo precipitate formation during spin coating in an atmosphere of upto at least about 55% relative humidity.
 10. A composition of claim 8wherein the solvent includes one or more methylbenzenes.
 11. Acomposition of claim 8 wherein the dipolar aprotic solvent isN-methylpyrrolidone.
 12. A composition of claim 1 wherein the2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane of said polyimide is2,2-bis[4-(3aminophenoxy)phenyl]hexafluoropropane, or a combination of2,2bis[4-(3-aminophenoxy)phenyl]hexafluoropropane and2,2-bis[4-(4aminophenoxy)phenyl]hexafluoropropane.